An operator of the handheld engine-driven working machine may determine an opening degree of a throttle valve by listening to a sound of an internal combustion engine so that the internal combustion engine is operated at an appropriate rotational speed. As such, the throttle valve may be fully opened or not fully (namely, partially) opened.
In such an engine-driven working machine, when the throttle valve is partially opened, it is difficult to stabilize the combustion due to a poor amount of air or fuel supplied to the engine so that the vibration of the engine-driven working machine is likely to become larger than that when the throttle valve is fully opened. If whether the throttle valve is partially or fully opened can be determined, the vibration due to the partially-opened throttle valve could be addressed.